The production of honeycomb articles, such as diesel particulate filters, often involves the application of ceramic cements (otherwise referred to as pastes or sealants) onto preformed honeycomb bodies. These cement compositions may be applied to form plugs, artificial skin (otherwise referred to as after-applied skin), or even to bond several smaller honeycomb segments together to make a larger honeycomb article. A common component of these cement compositions is colloidal silica. Colloidal silica is advantageous because it has low thermal expansion, provides strength without heating to high temperatures, and yet maintains strength at high temperatures (over 1000° C.). To that end, a conventional cement composition can consist of one or more ceramic powders, a liquid vehicle such as water, a water soluble polymer, typically methylcellulose, and an inorganic binder, typically the colloidal silica. The water and methylcellulose control the rheology of the paste and the colloidal silica is the inorganic binder that provides strength after the water is removed by drying and the methylcellulose is removed either by a heat-treatment step before use, during subsequent processing steps (such as washcoating), or during use.
One drawback associated with the use of colloidal silica recognized by the inventors is the migration of the very small silica particles (typically less than 30 nm). The migration can occur during the application of cement as the water which contains the silica particles, is pulled into the pores of the cellular ceramic honeycomb body by capillary forces and during drying, going from saturated to funicular to pendular pore structure as the water evaporates from the cement. This particle migration can lead to concentration gradients and resulting non-uniform properties of the final plug, artificial skin (higher silica concentration towards the outer, drying surface), or cement. These non-uniformities can lead to relatively lower strength due to lower levels of the colloidal silica binder, or possibly to non-uniform shrinkage. Further, if the cement is being applied to a ceramic composed of micro-cracked material, the colloidal silica particles may enter the micro-cracks which may lead to changed properties near the area of the applied paste. Typically, this change in properties is not desired.
One solution to this problem is to pre-fill the micro-cracks with organic material to prevent the colloidal particles from entering the cracks. This is known as passivation of the micro-cracks. While passivation may prevent property changes due to micro-crack filling, it does not solve the problem of non-uniform properties of the paste itself and also adds at least one process-step to pre-fill the microcracks.
Another issue with cement paste compositions containing colloidal silica is time-dependent rheology of the paste due to the finite-time the colloidal suspensions remain stable. As the colloidal suspension destabilizes, the particles bond together into larger agglomerates/flocs or networks of particles. Typically, this can result in increased yield point and viscosity of the paste over time, which may be undesirable in production settings.